select name from v$database; /查看数据库名/
    select instance_name from v$instance; /查看sid/

    select from dba_users; /查看数据库里面所有用户,前提是有dba权限的帐号,如sys,system/
    select     from all_users; /查看能管理的所有用户/
    select from user_users; /查看当前用户信息*/